This invention relates to a tone waveshape forming device used in an electronic musical instrument or the like device and, more particularly, to a device capable of smoothing the connection of a repetitive portion of the waveshape in forming a tone waveshape by repeatedly reading out the waveshape of plural periods stored in a memory.
U.S. Pat. No. 4,383,462 discloses an electronic musical instrument which aims at producing a tone of a high quality by prestoring a complete waveshape of a tone signal from rising to termination of sounding of the tone in a memory and reading out the waveshape therefrom. In the waveshape memory WM31 in FIG. 3 of this United States patent, a complete waveshape is stored and this complete waveshape is read out in response to a signal KD which represents a key depression timing. Such system in which the complete waveshape is stored is disadvantageous in that it requires a large memory capacity resulting in a high manufacturing cost and besides generation of a sustained tone is practically impossible.
In order to improve this point, it has been conceived to store a part of a waveshape of plural periods out of the complete sounding period in a waveshape memory and obtain a tone signal by repeatedly reading out the partial waveshape. In the above U.S. Pat. No. 4,383,462, an example of such improvement is shown in FIG. 6. A complete waveshape in the attack period is stored in the waveshape memory WM61 and at least one fundamental period of a tone waveshape is stored in the waveshape memory WM62. An attack waveshape is read out from the memory WM61 in response to the key depression (KD signal) and the tone waveshape of the fundamental period is repeatedly read out from the memory WM62 after completion of the readout of the attack waveshape (IMF signal) until the end of tone generation (DF signal). If, however, the waveshape portion of plural periods which are repeatedly read out were simply connected, the connection between the repetitive portions would be discontinuous and a clicking noise would result. For smoothing the connection between the repetitive portions, it has been attempted to take a waveshape portion as a repetitive portion out of an original waveshape, and the waveshape portion corresponds to a site where the first amplitude value and the last amplitude value of the repetitive portion are about the same. This can connect the repetitive portions at the same amplitude but there is no continuity in the amplitude before and after the connecting point with the result that the waveshape is bent at the connecting portion of the repetitive portions which is quite unnatural. Besides, this method involves a troublesome operation for taking the waveshape portion for the repeating purpose out of a proper site of the original waveshape.
A waveshape memory is commonly constructed by using as its memory element an integrated circuit memory having a specific memory capacity. In this memory, a waveshape should preferably be stored over the entire memory area of the memory for utilizing the memory area without waste. In that case, reading of a specific waveshape of plural periods is performed by returning a read address to a predetermined repeat address when the read address has reached last address of the memory area of the waveshape. In this case also, for smoothing the connection between the repetitive waveshapes, an address having nearly the same amplitude and phase of the last address is sought out and selected as the repeat address. This method, however, has the problem that the waveshape at the connecting portion is bent and therefore is unnatural. Besides, selection of a proper repeat address involves a complicated operation with a resulting high manufacturing cost. Since, particularly, the amplitude and phase of the last address are randomly determined by the memory size, seeking of a repeat address corresponding to such random amplitude and phase is extremely troublesome. Moreover, since waveshapes are all different from one another depending upon the tone pitch of a tone or the type of rhythm sound, such troublesome selection of the repeat address must be individually made for the respective different waveshapes, so that a very cumbersome and patient work is required.
A tone waveshape in the rise portion of a tone changes in a complicated manner, exhibiting a great difference from the relatively stable waveshape of a sustained portion. Accordingly, for generating a tone of a good quality, a waveshape of plural periods for the rise portion preferably is prepared separately from a waveshape of plural periods to be read out repeatedly and this rise portion is read out once and thereafter the repetitive portion is read out repeatedly. In this case too, some arrangement must be made to prevent the connection between the rise portion and the repetitive portion from becoming unnatural.
It is, therefore, an object of the present invention to realize, in a tone waveshape forming device in which the waveshape of plural periods of the rise portion is read once and thereafter the waveshape of plural periods of the repetitive portion is repeatedly read out, a smooth connection between the rise portion and the repetitive portion and also a smooth connection between the repetitive portions themselves.
It is another object of the invention to facilitate the selection of a waveshape of plural periods to be used as the repetitive portion.
As will be described later, the present invention employs the interpolation technique for achieving the above objects of the invention. Known in the art of tone waveshape generation utilizing interpolation is a technique disclosed in U.S. Pat. No. 4,036,096. In this patent, the same waveshape is stored in a coarse sample interval in basic value memories 11 and 12 thereby to achieve reduction in the memory capacity. Two adjacent sample points stored in the memories are interpolated in accordance with the following equation to obtain a sample value Y: EQU Y=A+(B-A).multidot.X(C)
Where A and B represent sample values of the two adjacent sample points and X(C) represents interpolation function according to which X(C)=0 when C=0, X(C)=1 when C=1 and 0.ltoreq.X(C).ltoreq.1 when 0.ltoreq.C.ltoreq.1. For example, X(C)=1/2 (1-cos .tau.c). Thus, the known interpolation is one between the adjacent sample points. In the present invention, however, the interpolation is one which smooths the amplitude change of the waveshape in the interpolation section.